Validity of RR Lyrae epochs

I have another question concerning my Master's thesis project at Lund Observatory. It's about evaluating the how useful some old RR Lyrae epochs are. I hope the photometry forum is suitable for such a question.

I have spectra of two RR Lyrae stars in M 22, and I need to calculate their phases at the time when my spectra of them were obtained. The epochs I have are from Wehlau & Hogg 1978 (AJ vol. 83 no. 8), and they are from mid-1975. I know that the evolutionary changes of the period of a RR Lyrae star is quite slow (even on a scale of 10^6 years), but could something else make epochs invalid as time passes? Can the O-C diagram be used to estimate such things?

There are several reasons why one might run into trouble predicting current epochs of maximum light based upon ephemerides published in the mid-1970s. First, the original periods might not have been quite correct, and since many pulsation cycles have elapsed since the 1970s, the cumulative error might today be quite large. Second, signficant period changes can occur on a timescale of decades. Although the theoretical period changes of RR Lyrae stars can be quite small for much of their life on the horizontal branch, the observed period change rates can often be larger. Some RR Lyrae stars have shown no period changes in observations spanning decades but that is not true for all. The variety of period change behavior can be seen by examining the O-C diagrams in, for example, the GEOS database at http://rr-lyr.ast.obs-mip.fr/ . An O-C diagram extending to recent dates would allow the old ephmerides to be extended, but one cannot count on past O-C trends to continue into the future in all cases. Your phases will be on much firmer ground if you can find some more recent ephemerides for the M22 RR Lyrae.

I think you should definitely look if there is newer photometry of your targets to be able to get up to date ephemerides. If there isn't any and you have time and resources, then maybe thinking of getting new photometry for more reliable light curve phasing would be good. That's likely not feasible within a masters project but could be relevant if you continue working on these targets after that.

At least calculate the cumulative errors of the maximum timings between the zero epochs of the published ephemerides and the dates of your spectra. That will give you some initial idea of the usability of the ephemerides. If the timing errors are a significant fraction of the pulsation periods of the stars, the old ephemerides won't do too much good for you even if the stars' pulsations would have stayed absolutely stable up to today.